1. Field of the Invention
The present invention is directed to a process for converting methane to higher hydrocarbons, especially ethylene. More specifically, the present invention is directed to a process for converting methane to higher hydrocarbons, especially ethylene, utilizing a catalytic system which includes a Group IB metal, a chloride of an alkali metal, an alkaline earth metal or a rare earth metal of the lanthanide series and a volatile halide.
2. Background of the Prior Art
New and improved processes for synthesizing higher hydrocarbons, that is, hydrocarbons containing at least two carbon atoms, especially ethylene, from methane have been a continuing aim of those skilled in the art. This is to be expected in that while methane is readily available at low cost, the higher hydrocarbon product of such processes, such as ethylene, is far more costly and useful. Ethylene and other higher hydrocarbons are valuably utilized as intermediates in the production of liquid fuels, plastics, fibers, solvents and a plurality of other organic compounds used in the chemical process industries.
Because of this obvious attractiveness of employing low cost and readily available methane in the synthesis of higher value higher hydrocarbons, a multiplicity of processes utilizing methane as a source of these higher hydrocarbons have been developed. Among the more relevant of these processes disclosed in the prior art are processes utilizing metal oxide catalysts. One such teaching is G.E. Keller et al., Journal of Catalysis, Vol. 73, 9-19 (1982). Keller et al. describes a method for synthesizing ethylene by the oxidative coupling of methane In this method methane is converted, in a gas phase catalytic reaction, to ethylene and ethane at atmospheric pressure and a temperature of from 500.degree. C. to 1000.degree. C. The catalyst used in this reaction is selected from the group consisting of an oxide of tin, lead, bismuth, antimony, tellurium, cadmium and manganese supported on alumina. Of particular interest is the conclusion that many metal oxides, including silver and copper, show little or no activity as catalysts in this reaction.
A more recent disclosure, S. Imamura et al. Chem Express, Vol. 2 (1), 49-52 (1987), discloses that silver oxide in admixture with lead oxide catalyzes the oxidative dimerization of methane to ethylene and ethane.
Yet a third reference directed to the use of oxides of Group IB metals as catalysts in the conversion of methane is the PCT application, WO 86/07351. That patent application discloses a process and catalyst for the synthesis of hydrogen, ethylene, ethane and higher hydrocarbons from methane in the presence of oxygen. The catalyst utilized in this process is an oxide of a metal selected from the group consisting of a Group IIA metal, a Group IIIA metal, a lanthanide series metal excluding cerium and mixtures thereof. Optionally, a promoter for the catalyst, an oxide of a metal selected from a Group IA metal, a Group IIA metal, a Group IIIA metal, a lanthanide series metal, a Group IVB metal, a Group VB metal, a Group IB metal or mixtures thereof may also be utilized.
In addition, Group IB metals in other than the oxide state have been suggested for use in the synthesis of ethylene and other hydrocarbons from methane. One such teaching is included in German Pat. Publication 3,503,664. The '664 patent publication claims the use of silver halides, among other materials, as catalysts in the catalytic oxidation of methane with oxygen at elevated temperatures, i.e. 600.degree. C., to 1,000.degree. C., to yield ethylene and ethane while co-feeding hydrogen chloride therewith.
It is emphasized that an discussion of processes employing Group IB metals as catalysts in the conversion of methane to higher hydrocarbons must include the disclosure in U.S. Pat. 4,684,800. The '800 patent teaches a method for converting methane to higher hydrocarbons by contacting methane, an oxygen-containing gas and a reducible metal oxide in the presence of at least one promoter selected from the group consisting of halogens and halogen compounds. The preferred metals of the reducible metal oxide comprise manganese, tin, indium, germanium, antimony, lead, bismuth and mixtures thereof. In addition, cerium, praseodymium or terbium rare earth metal oxide can be utilized as can reducible iron or ruthenium oxides when associated with an alkali metal component and/or an alkaline earth metal component.
Of particular interest is the disclosure in the '800 patent requiring that the reaction occur in the substantial absence of a catalytically effective amount of nickel, noble metals or compounds thereof. Specifically, the '800 patent states that the presence of Group IB metals silver and gold as well as nickel, rhodium, palladium, osmium, iridium and platinum have a deleterious catalytic effect on this reaction. The disclosure states that these metals when contacted with methane at reaction temperature i.e., 800.degree. C. to 900.degree. C., tend to promote coke formation as well as combustion products rather than the desired products, ethylene and higher hydrocarbons.
In summary, the prior art description relating to the use of a Group IB metal as a catalyst in the catalytic transformation of methane to higher hydrocarbons, especially ethylene, is none too promising. The weight of the teaching of the references suggest that these metals do not present a fruitful avenue for future research to bring to fruition this desirable transformation. However, the earlier remarks establishing the strong desirability of providing new processes to effectuate this reaction urge workers in this art to examine and reevaluate all possible catalytic agents that may effectuate the reaction of methane to produce ethylene.